> Le 29 oct. 2015 à 03:51, Boris Zbarsky <[email protected]> a écrit : > > I was just implementing subclassing of Promise in Gecko, when I realized that > given a Promise subclass MyPromise these two calls: > > MyPromise.race([]) > MyPromise.all([]) > > will take MyPromise[@@species] into account when creating the return value, > but these two calls: > > MyPromise.resolve() > MyPromise.reject() > > will not; they will invoke MyPromise itself, not MyPromise[@@species]. > > This is because > http://www.ecma-international.org/ecma-262/6.0/#sec-promise.all and > http://www.ecma-international.org/ecma-262/6.0/#sec-promise.race do the whole > @@species thing but > http://www.ecma-international.org/ecma-262/6.0/#sec-promise.reject and > http://www.ecma-international.org/ecma-262/6.0/#sec-promise.resolve do not. > > Is this behavior intentional? If so, I'd really like to understand the > reason for it. > > -Boris
Relevant discussion: https://esdiscuss.org/topic/performpromiseall <https://esdiscuss.org/topic/performpromiseall> As I understand, the specified behaviour is an accident of history. Previously, `Promise.resolve()` and `Promise.reject()` used the species pattern in order to determine the constructor to be used, just as `Promise.all()` and `Promise.race()`. Then, at the last minute, it was decided that it wasn’t the best semantics for `Promise.resolve()`, and it was corrected. For `Promise.all()` and `Promise.race()`, the motivation wasn’t stronger than the lack of time. Should it be corrected before @@species is widely implemented? I think so. The arguments I give are: * Overall consistency in the language. Except for the two offending Promise static methods, all uses of `@@species` in the ES2015 spec (15 uses) are for the following pattern: Starting from one instance, one constructs a derived object for that instance. (The effective lookup of the @@species property is factored in the SpiecesConstructor and ArraySpeciesCreate abstract operations.) * Also, in static methods like `Promise.all` and `Promise.race`, a constructor is explicitly provided by the user: simply use it. Compare with what is done for arrays: Array.from(someArray) // use the Array constructor explicitly provided. Array.of(x, y, z) // ditto someArray.slice(0) // compute the constructor to be used through some algorithm involving @@species. [].concat(x, y, z) // ditto —Claude
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